In a hemodiafiltration, or dialysis treatment, system, to remove substances that are eliminated with urine, a patient's blood is sent in an extracorporeal circulation system through a chamber of a dialyzer subdivided into two chambers by a semipermeable membrane, while a dialysis fluid flows through the other chamber. Frequently an arteriovenous fistula is created for access to the vascular system, but it is also possible to insert an implant.
Blood is taken from the patient through an arterial needle which is connected to the arterial blood line of the extracorporeal circulation and is supplied to the patient again through a venous needle connected to the venous blood line.
German Patent Application No. 2,838,414 describes a dialysis machine having a volumetric balancing device. The balancing device consists of two chambers subdivided by a displaceable element, with each chamber having an inlet line for fresh dialysis fluid and a drain line connected to an outlet for spent dialysis fluid. Cutoff valves which are driven and switched by a control unit are arranged in the inlet and drain lines. A pump is arranged in the dialysis fluid path between the dialyzer and the balancing device for conveying the spent dialysis fluid.
For the safety of the patient during the dialysis treatment, monitoring of the vascular access is of crucial importance. Thus, if the venous needle slips out and this error is not detected immediately, it involves great blood loss for the patient.
Protective systems for monitoring a vascular access are known from the area of infusion technology. European Patent Application No. 328,163 describes an infusion apparatus with a pressure transducer in the infusion line for detecting the patient's heart beats in the infusion line if the needle has access to the vascular system. Thus, a faulty vascular access is detected by the fact that the heart beats are no longer measured as pressure pulses in the infusion line.
European Patent Application No. 328,162 describes an infusion apparatus where the pressure pulses generated by the infusion pump in the infusion line are monitored. If a needle slips out, it is detected by a change in shape of the pressure pulses.
A previous dialysis machine with a device for monitoring a vascular access has a pressure transducer arranged in the venous blood line. The pressure transducer detects a pressure drop that occurs when the needle slips out. A study of venous pressure monitoring with dialysis equipment has shown, however, that monitoring of venous return pressure as a protective system to prevent blood loss into the environment can fail when the needle slips out.
International Patent Application No. WO 97/10013 describes a dialysis machine with a monitoring system which monitors in the venous blood line the pressure pulses generated by the blood pump in the arterial blood line. This protective system has the disadvantages that the pressure pulses are generated in the extracorporeal blood circulation. Appropriate equipment needs to be provided on the blood side for this purpose.